1. Field of the Invention
The present invention relates to an information processing apparatus, an image compression method, and a storage medium. In particular, the present invention relates to a method useful for compressing a non-compressed image and storing the compressed image data, and decompressing the compressed image data.
2. Description of the Related Art
An image forming apparatus, such as a printer, which prints a color image at a high resolution and high gradation, generally compresses image data. In an image forming apparatus having a development unit drum transfer type image forming apparatus, which is a mainstream apparatus in the field of printing, it is necessary to continue transmitting data of each page to a fixing device at a predetermined timing.
If image data of high resolution and high gradation is input, because its data size is large, it takes a long time to process data within the image forming apparatus. In addition, because the size of a hard disk drive (HDD) and a memory is limited, the image forming apparatus compresses image data to reduce the data size of data to be processed. In addition, by using dedicated hardware, the image forming apparatus can decompress image data that has been compressed to a predetermined size in a short time.
In a conventional image forming apparatus discussed in Japanese Patent Application Laid-Open No. 05-64001, in order to implement printing at a high printing speed, page data is compressed to a predetermined small size, which is determined for each compression ratio, by changing the compression ratio.
In a large sense, a lossless compression method and a lossy compression method are used as the method for compressing an image executed in the image forming apparatus like this. In printing a color image, the image forming apparatus uses the lossy compression method, such as Joint Photographic Experts Group (JPEG), which can implement a high compression ratio. When such a lossy compression method is used, the image quality of resulting data may be low because the compression ratio thereof is high. On the other hand, in generating data having a high image quality, it is necessary to apply a low compression ratio.
Accordingly, in a conventional image forming system, in order to achieve a high image quality, image data is repeatedly compressed with the compression ratio being gradually increased starting from a low compression ratio until the amount (size) of image data after compression becomes a predetermined size or smaller. Thus, the balance between the data amount and image quality is achieved.
If data for variable data printing (VDP) has been input, the image forming system like this processes its page data in the unit of an object (data), such as a text or an image. “VDP” refers to a printing method in which a user who desires to print a document produces a print product uniquely produced for each customer, who receives the print product.
The image forming system provides a print product optimally produced for each customer by generating page data based on a combination of data previously rasterized by using a raster image processor (RIP) and data for each customer.
When VDP data is input, the image forming system stores data reusable within the same document as a reusable object (reusable data). In printing the VDP data that uses the reusable data again, the image forming system RIP-processes variable data only, and generates page data by combining the RIP-processed variable data and the stored reusable data. As described above, it becomes unnecessary for the conventional image forming system to execute RIP processing on the reusable data. Thus, the print time can be reduced.
However, in printing VDP data in the image forming system capable of compressing image data, if page data including reusable data is compressed again and again, the image forming system also re-compresses another page data that utilizes the reusable data. If data is re-compressed, a long processing time becomes necessary for printing the data. Accordingly, in this case, the printing performance of the image forming system may be affected.